For the description of different fluidized beds and especially circulating fluidized beds, reference is made to L. Reh "Fluidized Bed Processing" Chemical Engineering Progress, Vol. 67, No. 2, February 1971, pages 58-63.
Reference can also be made to the Swedish Patent specification 355 956, relating to a method for carrying out endothermic processes, wherein solid material is treated in a reaction zone consisting of a heavily expanding fluidized bed, i.e. a circulating fluidized bed. A major portion of the solid material is discharged together with gas in a upper part of the fluidized bed reactor. A portion of the heat quantity required for the endothermic process is supplied to the fluidized bed by a gas having a temperature of at least 300.degree. C. The solid material discharged at the upper part of the reactor is separated from the gas in a cyclone separator and is at least partially refluxed to the fluidized bed. This known method is useful, i.a. in reduction processes, e.g. the reduction of gypsum, and chemical processes at high temperature, e.g. the oxidation of ilmenite.
It has already been proposed to produce a molten carbon-containing metal from a metal oxide, e.g. raw iron from iron oxide, by pre-reduction of the metal oxide followed by a smelting reduction, the heat requirement of which is provided to a major extent by electrical heating (Swedish Patent No. 7305753-1, publication number 393 816). An additional fuel, intended for the production of electrical energy, is used here for pre-heating, pre-reduction and other pre-treatment of the raw materials for the smelting reduction, and the remaining heat content in this fuel, as well as in the exhaust gases from the smelting reduction, are used for generating electrical energy which is utilized for the smelting reduction.
The pre-reduction process as described above, i.e. reduction of finely divided iron oxide material mixed with finely-divided solid carbonaceous material, has been further developed in the way described in the Swedish Patent specification No. 7403145-1 (publication number 384 225), a circulating fluidized bed being utilized and a degree of reduction up to 85% being obtained. In this known method the circulating fluidized bed is maintained in a vertically elongate reaction zone by adding to said reaction zone adjusted flows of finely divided solid materials, and possibly also a liquid carbonaceous material, and a gas containing molecular oxygen, and by subjecting departing gas and solid material from the reaction zone to a separation process, separated solid material being recycled into the reaction zone. The finely devided iron oxide material, the carbonaceous material and the gas containing the molecular oxygen are supplied to an intermediate portion of the reaction zone, and the flow of carbon supplied is controlled so that a proportion of coke is always maintained in the bed such that disturbance of the fluidization due to stickiness is prevented. Gas and solid material departing from the reaction zone are taken out from its upper part, the separated solid material being recycled to the intermediate portion of the reaction zone. Solid material containing wholly or partially reduced iron oxide is discharged from the lower part of the reactor zone.
The latter mode of operation has signified a clear improvement of pre-reduction possibilities but it has been found that the result has not been entirely satisfactory. Difficulties have occurred, i.a. due to the iron oxide particles and carbon particles segregating so that the lower part of the reactor is given a considerably greater content of iron oxide than the upper part, while the situation is the opposite for the carbonaceous material. This segregation has resulted in a deteriorated reduction process and in functional disturbances in the plant. The material has furthermore adhered to the air supply nozzles and to the furnace wall around said nozzles.